The present invention relates to a lubrication fluid supply system for a mechanism. It relates more particularly to the lubrication of a rotary bearing on board a spacecraft, such as a satellite, a rocket, or a space vessel.
The precise control and good monitoring of oil supply, and more generally of lubrication fluid supply, in on-board mechanisms such as rolling bearings, are very critical for this type of applications. An insufficient supply may namely cause a degradation of the system to be lubricated, and even its jamming in the worst case. On the other hand, an excess supply of lubricant may cause the braking moment to be too large, which may additionally induce vibrations. This also can cause an increase in the pollution rate in the neighboring (optical, etc.) instruments, as a result of an evaporation and condensation phenomenon.
Two main categories of lubricating systems may be distinguished, based on their operating modes: the "passive systems" and the "active systems". A priori, an "ideal system" should belong to the second category. Furthermore, a feedback loop controller can be associated with it.
More precisely, the "passive" systems supply oil or a lubrication fluid, without any external controller. Such systems can for instance be based on the effect of centrifugal forces. To clarify the notions, considering the case of an inertia flywheel (used for instance in a gyroscope), it is an obvious inconvenience that the supply is dependent upon the rotation speed. A further inconvenience is that the intrinsic "feedback loop" action is opposite to the desirable action. Namely, if the flywheel tends to slow down, precisely because of the amount of lubricant supplied to a mechanism being insufficient, this causes a further reduction of the amount of oil supplied. In other words, this phenomenon is cumulative.
A correct operation, on the other hand, can be monitored in the active systems. Such systems are based on different principles.
A first type of systems comprises a pressurized, lubricant filled, tank. The tank is closed by a valve. The latter is controlled by a spring made of a shape memory alloy, or "SMA", which monitors the amount of lubricant flowing out of the tank, based on a pressure difference between the tank interior and the outside environment.
An other pressurized tank system, using a "Kaiser Eckel" type solenoid valve, is described in an article by Dennis W. Smith and Fred L. Hooper : "POSITIVE LUBRIFICATION SYSTEM", published in "The 24th Aerospace Mechanisms symposium", pp 243-258.
A second type of systems implements a micro-pump and works according to the same principle as an ordinary pump. An internal chamber is alternately filled by creating a depression and emptied by creating an overpressure. To this purpose, a high frequency actuator member, most frequently of a piezo-electric or electrostatic type, is used. The existing micro-pumps have sizes in the millimeter, or even micrometer ranges, and can be fabricated by silicon etching or stereolithography, for instance.
Those two lubrication system types, described in a more detailed manner hereinafter, however suffer from serious inconveniences, which also will be clarified.
An other system, based on an entirely different principle, was developed and described in an article of L. M. Dormant and S. Feuerstein : "Nylon Pore System", published in the "Journal of Spacecraft and Rockets", volume 13, No 5, May 1976, pp 306-309. This system implements oil impregnated, porous nylon blocks. Controlled heating of those blocks causes a certain amount of oil to be expelled, due to an expansion effect. It however was demonstrated that this system, because of capillarity effects, does not correctly operate.
In a more general way, no prior art system has been entirely satisfactory. Furthermore, presently existing systems, with the exception of the micro-pump system, have relatively large sizes.
There consequently is a need for a lubrication system offering both a small size and a very high reliability.
To clarify the notions, and without this limiting in any way the scope of the invention, the case of the lubrication of an inertia flywheel bearing (used for instance in a gyroscope) will be exposed herein. This application is interesting since it evidences the encountered difficulties. This device is used in most positioning control systems and the flywheel, for long-term missions, should revolve for periods of time currently longer than ten years.
Furthermore, the typical required performances and the environmental conditions are very severe. An non-exhaustive listing is given hereafter:
high vacuum: 10.sup.-3 to 10.sup.-4 bars (100 Pa to 10 Pa); PA1 operation in a wide temperature range: from -40.degree. C. to +65.degree. C. PA1 capacity to operate with very viscous lubricants: up to 20 Stokes (2 10.sup.-3 m.sup.2 /s), as a consequence of low temperatures); PA1 a geometry allowing implementing the lubrication system close to the bearing: geometry typically inscribed in a 8.times.20.times.20 mm volume (but with a complex shape, irreductible to a simple cuboid); PA1 a fluid volume to be delivered typically in the range of 2 to 3 cm.sup.3 ; PA1 a monthly dose typically in the range of 1 to 5 mm.sup.3 ; PA1 a 10 to 20 years duration, for long term missions; PA1 a capacity to operate under a 0 g (0 m/s.sup.2) acceleration; PA1 an electric control interface needing no high voltage power supply, avoiding a.c. voltage, and only requiring a minimal power consumption; PA1 a high resistance to vibrations while being launched into orbit, with the following typical levels: 50 g.sub.n (490 m/s.sup.2) in the quasi-static conditions; 30 efficient g.sub.n (294 m/s.sup.2) in the random conditions and 200 g.sub.n (1960 m/s.sup.2) in cases of shocks of a typical 0.5 ms duration. PA1 a resistance to very high-level radiations, typically 100 krad. PA1 absence of any pressurized tank PA1 no valve, just a simple piston which is not permanently exposed to pressure; PA1 a very good monitoring of the amount of oil (or more generally of the lubrication fluid) being delivered, due to the operating mode implying the supply of volumetric doses; PA1 independence from the (temperature dependent) viscosity of the fluid used, and quasi-independence from the most important parameters (time, temperature, pressure); PA1 simple electric interface (no need for either a.c. voltages or high voltage power supply, or feedback control, or "intelligent" control circuits).